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1 HNRS 227 Fall 2008 Chapter 4 Heat and Temperature presented by Prof. Geller Do You Remember These? Units of length, mass and time, and metric Prefixes Density and its units The Scientific Method Speed, velocity, acceleration Force Falling objects Newton s Laws of Motion Newton s Law of Universal Gravity Work Potential Energy and Kinetic Energy Conversion of Energy Types/Sources of Energy iclicker Question What is the result if you add to 10 15? A B C D E 2 x iclicker Question Which unit of the following is the largest? A Kilometer B Astronomical Unit (AU) C Light Year (ly) D Parsec (pc) E Gigameter iclicker Question What is acceleration? A The change in distance per change in time. B The change in position per unit of time. C The ratio of the change in velocity per change in time. D The change of time per unit of length. iclicker Question What happens to the velocity and acceleration of an object in free fall? A The velocity decreases as the acceleration remains the same. B The velocity increases as the acceleration remains the same. C The velocity increases and the acceleration decreases. D The velocity increases and the acceleration increases. E Both velocity and acceleration decrease. 1

2 iclicker Question Neglecting air resistance, what are the forces acting on a bullet after it has left the barrel of a rifle? A The force of air acting up and gravity acting down. B Only the force of gravity acting straight down. C There are no forces acting at this point. D All of the above are true. A kwhr is A a unit of work B a unit of energy C a unit of power D More than one of the above is true. A kwhr is a unit of work, and since energy is the ability to do work, it is also a unit of energy. In terms of units, a watt is a joule per second, and an hour, as a second, is a unit of time. The time units cancel, leaving a unit of a joule, which can be used to measure either work or energy. What happens to the kinetic energy of a falling book when the book hits the floor? A The kinetic energy is destroyed. B The kinetic energy is converted to heat only. C The kinetic energy is converted to heat and sound. The magnitude of the force that a baseball player exerts with a 5 kilogram baseball bat on a 0.2 kilogram ball is measured to be 50 Newtons. What is the magnitude of the force that the ball exerts on the bat? A. 6 Newtons B. 10 Newtons C. 50 Newtons D. 60 Newtons E. 1.0 Newtons F2 Box F1 In the diagram above, a box is on a frictionless horizontal surface with forces F1 and F2 acting as shown. If the magnitude of F1 is greater than the magnitude of F2, then the box is A. moving at constant speed in the direction of F1 B. moving at constant speed in the direction of F2 C. accelerating in the direction of F1 D. accelerating in the direction of F2 E. not moving at all. When a satellite is a distance R from the center of Earth, the force due to gravity on the satellite is F. What is the force due to gravity on the satellite when its distance from the center of the Earth is 2R? A. 9 F B. 4 F C. F / 4 D. F / 9 E. F / 16 2

3 Which of the following best describes the law of conservation of energy? A. Energy must not be used up faster than it is created or the supply will run out. B. Energy can be neither created nor destroyed. C. Energy is conserved because it is easily destroyed. D. Conservation is a law describing how to destroy matter. E. Energy conservation is a law recently passed by Congress. What is the power consumed by a 100 Joules energy source in 4 seconds? A. 400 Watts B. 100 Watts C. 200 Watts D. 50 Watts E. 25 Watts Main Concepts for Chapter 4 Kinetic Molecular Theory Temperature Heat Phases of matter Thermodynamics 1st Law of Thermodynamics In an isolated system, the total amount of energy, including heat energy, is conserved. ENERGY IS CONSERVED Temperature A relative term reflecting how vigorously atoms in a substance are moving and colliding Alternative definition the average kinetic energy of the molecules in a region Temperature Units Fahrenheit Celsius Kelvin Where is absolute zero? At what temperature does water freeze? Boil? 3

4 Heat Heat is a form of energy the energy flows from a warmer object to a cooler object Units of heat calorie amount of temperature needed to raise temperature of 1 gram of water 1 degree Celsius kilocalorie (kcal or Calorie) amount of temperature needed to raise temperature of 1 kg of water 1 degree Celsius Specific Heat Ability of a material to absorb heat energy proportional to mass proportional to change in temperature Q = m*c*δt (note: not in textbook) c is the SPECIFIC HEAT of the substance amount of energy needed to increase temperature of 1 gram of substance 1 degree Celsius Transfer of Heat Conduction movement of heat by collisions between vibrating atoms or molecules Convection transfer of heat by the physical motion of masses cooler liquids or gasses descend while warmer liquids or gasses rise Phases and Phase Diagram (NOT IN TEXT but IMPORTANT) 2nd Law of Thermodynamics Two key components heat flows from a warmer body to a cooler body entropy increases remains constant or increases in time 4

5 Which is true about temperature and heat. A Temperature is a measure of the average kinetic energy of the molecules of a substance. B Heat is the total internal energy of the molecules involved in an energy transfer. C Both A and B above are true. D Neither A nor B above are true. As the temperature of a solid increases, the vibrations of the individual molecules become larger. When these vibrations become larger, the average distance between the molecules increases to accommodate these larger oscillations, and the solid expands. In a liquid or a gas, the individual molecules move faster as the temperature increases, and the collisions between individual molecules become more violent. Since the molecules are moving faster, they move farther apart as they travel a larger distance in the time between collisions. A This explains why most materials become less dense as their temperature is increased. B This explains why most materials become more dense as their temperature is increased. Would the tight packing of more insulation, such as glass wool, in an enclosed space increase or decrease the insulation value? A Increase the insulation value. B Decrease the insulation value. A true vacuum bottle has a double-walled, silvered bottle with the air removed from the space between the walls. Describe how this design keeps food hot or cold by dealing with conduction, convection and radiation. Tight packing would tend to decrease the insulation value of glass wool because it would squeeze the wool together and give the heat more paths to travel. It is the presence of many small pockets of air, with unattached molecules, that gives glass wool and other similar insulation materials their insulating properties. The vacuum between the walls prevents heat transfer by means of convection or conduction, while the silvered walls reflect radiated energy back into (or away from) the food, preventing energy transfer by radiation. Cooler air is found in low valleys on calm nights. A True B False True. Cooler air is denser than warmer air. This denser air weighs more per volume than the warmer air and pushes the warmer air out of the way as it sinks down to its lowest level. The warmer, less dense air sits on top of the cooler air because it weighs less per volume. Is air a good insulator? A Yes B No Air is a good insulator because it is not very dense and conduction is not very efficient at transferring energy because the molecules are much farther apart than they are in solids or liquids. 5

6 Can you explain the meaning of the mechanical equivalent of heat. What do people really mean when they say that a certain food has a lot of Calories? Mechanical energy can be converted to heat as it changes from external mechanical energy to the internal kinetic energy of the molecules. A given quantity of mechanical energy always yields a known amount of heat. When people refer to the Calorie content of food, they are referring to the amount of chemical energy available from the food. One way to measure the chemical energy of foodstuff is to find out how much heat is released by complete oxidation. A Calorie (kcal) is a measure of the heat release and thus is a measure of the chemical energy released. A piece of metal feels cooler than a piece of wood at the same temperature. A True B False Can you explain how latent heat of fusion and latent heat of vaporization are hidden. True. The metal is more efficient at conducting heat away from your hand than wood, so it feels cooler because your hand senses heat leaving your body. The latent heats of fusion and vaporization go into or are released from internal energy during a phase transition. There is no temperature change associated with these heat transfers, so they are hidden. The condensation of water vapor on a bathroom mirror warms the bathroom. A B True False True. Condensation occurs when more vapor molecules are returning to the liquid state than are leaving the liquid state. When a water vapor molecule joins a group of liquid water molecules, it has to give up its latent heat of vaporization. This heat is transferred to the surrounding air molecules such as the air in the bathroom. Which provides more cooling for a styrofoam cooler? A One with 10 pounds of ice at 0 degrees C B One with 10 pounds of ice water at 0 degrees C? The 10 pounds of ice (A) provide more cooling because as the ice undergoes the phase change into water, it absorbs heat. Ten pounds of ice water simply absorbs heat according to the value of its specific heat until it reaches room temperature and therefore absorbs less heat. 6

7 Consider that a glass filled with a cold beverage seems to sweat. Would you expect more sweating inside a house during the summer or during the winter? A Summer B Winter Water condenses out of the air onto the cooler surface of a glass because the air near the glass is cooled, lowering its temperature to the dew point. Since the warmer air can hold more water vapor in the summer, it would have more water vapor to condense. Therefore, you would expect more condensation in the summer. A burn from steam at 100 degrees C is more severe than a burn from 100 degrees C water. A True B False True. One hundred degree Celsius steam contains more energy (540 cal/g) than 100 C water, so the steam burn would be more severe. Relative humidity typically increases after sunset. A True B False True. Cooling of air reduces the capacity of air to hold water vapor. Relative humidity is a ratio of water in the air to how much water it can hold. Thus a decrease of capacity increases the relative humidity, even when the amount of water vapor in the air is constant. Question A 1.0 kg metal head of a geology hammer strikes a solid rock with a velocity of 5.0 meters per second. Assuming that all of the energy is retained by the hammer head, how much will the temperature increase? (specific heat of hammer head is 0.11 kcal/kgc) Answer Question 4. Q = ΔKE and ΔKE = 1 2 mv2 Q = 1 2 mv 2 1 Q = mcδt mcδt = 1 2 mv 2 2 mv cal J ΔT = mc T = kg) 2 s ( 1.00 cal) 1.00 kcal ( 1.0 kg) 0.11 kcal kg C 1, cal = 0.03 C Lead is a soft, dense metal with a specific heat of kcal/kgc, a melting point of o C and a heat of fusion of 5.5 kcal/kg. How much heat must be provided to melt a kg sample of lead with a temperature of 20.0 o C? 7

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